Chlorinated hydrocarbons are emitted to the atmosphere from their widespread use as solvents in diverse industrial, commercial and domestic applications. Dichloromethane (CH₂Cl₂), trichloroethene, (C₂HCl₃) and tetrachloroethene (C₂Cl₄) are three important chlorinated solvents that are not regulated by the Montreal Protocol. Due to their short atmospheric lifetimes they have low impacts on stratospheric ozone and are removed from the atmosphere through oxidation by tropospheric OH, in the order of their atmospheric lifetimes C₂HCl₃-5 days, C₂Cl₄-100 days, and CH₂Cl₂-140 days. However, they are classed as hazardous air pollutants and toxic volatile organic compounds in regional air quality inventories.

GC-MS observations of dichloromethane (CH₂Cl₂), trichloroethene (C₂HCl₃), and tetrachloroethene (C₂Cl₄), have been recorded at two AGAGE station, Mace Head, Ireland and Cape Grim, Tasmania. However, trichloroethene is usually below the limit of detection at Cape Grim except during pollution episodes.

At Mace Head CH₂Cl₂ shows a downward trend from 1995-2004 of 0.7 ± 0.2 ppt yr^-1 (ppt: expressed as dry mole fractions in 10¹²), although from 1998-2004 the decrease has been only 0.3 ± 0.1ppt yr^-1.  Conversely, there has been a small but significant growth of 0.05 ± 0.01 ppt yr^-1 in CH₂Cl₂ at Cape Grim (1998-2004). The time series for C₂HCl₃ and C₂Cl₄ are relatively short for accurate trend analyses, however, we observe a small but significant decline in C₂Cl₄. (0.18 ± 0.05 ppt yr^-1) at Mace Head.

European emissions inferred from AGAGE measurements of these three solvents are compared to recent estimates from industry data, and show general agreement for C₂HCl₃. Emissions estimated from observations are lower than industry emission estimates for C₂Cl₄ and much lower in the case of CH₂Cl₂.  A study of wildfires in Tasmania, uncontaminated by urban emissions, suggests that the biomass burning source of CH₂Cl₂ may have been previously overestimated.

There are a number of possible explanations for the disagreement between the European emission estimates for CH₂Cl₂ by the NAME model and industry. The geographical areas covered by the two estimates are not identical although both methods include the major emitting countries. The calibration scale (UB98) for CH₂Cl₂ is based on a single primary calibration mixture. Recent comparisons of calibrated AGAGE CH₂Cl₂in situ measurements with NOAA flask data at Mace Head show an absolute calibration difference of 8.8 ± 4%, with NOAA data higher. Reporting the AGAGE measurements on the NOAA scale would improve agreement between the two emission estimates in most years.

Emissions from Europe may be overstated by the industry estimates in a number of ways despite the fact that global emissions from the same database are consistent with our 12-box model results. Emissions are derived from consumption (sales) data and are assumed to occur in the same geographical area. Material shipped directly by producers is assumed as consumption at the destination, but significant exports from Europe of the solvents (by chemical merchants, not producers) or in equipment containing the solvents would result in the true emissions in Europe being lower than derived from consumption data.

There may be seasonal variability in the emissions and the relative loss rates of each individual solvent during transport are uncertain. Furthermore, halocarbons dissolved in surface water might be transported over a significant distance before temperature changes might cause them to be re-emitted to the atmosphere. Notably, of the three solvents CH₂Cl₂ has the highest water solubility [13.2g/liter @20°C].

There may also be unaccounted for sinks, such as hydrolysis, dry deposition and biodegradation which could perturb the relative emission ratios from the primary source regions. No single factor appears to account for the inconsistencies between industry-derived, NAME model-derived and CO inventory estimated emissions. In order to resolve some of these questions high frequency measurements from additional sites in Europe are required.

For reprints and further information, please contact Peter Simmonds: PETERGSIMMONDS@aol.com

Article: Observations of dichloromethane, trichloroethene and tetrachloroethene from the AGAGE stations at Cape Grim, Tasmania, and Mace Head, Ireland. Simmonds, P.G., A.J. Manning, D. M. Cunnold, P.J. Fraser, A. McCulloch, S. O'Doherty, P.B. Krummel, R.H.J. Wang, L.W. Porter, R.G. Derwent, B. Greally, P. Salameh, B.R. Miller, R.G. Prinn, and R.F. Weiss (2006), Journal of Geophysical Research: Atmospheres, 111: D18304 (doi:10.1029/2006JD007082).